1. Field of the Invention
The present invention relates to an air conditioner having two compressors, and more particularly to an apparatus for controlling the capacity of an air conditioner having a pair of first and second compressors, in which a 3-way or 4-way direction-switching member and a low-pressure equalizing solenoid valve are provided at a refrigerant path of the air conditioner so that the compression capacity of the air conditioner is adjusted into three stages of 100%, 60%, and 40% using the first and second compressors to enable easy variable-capacity operation, thereby considerably reducing energy consumption and preventing wear of the first and second compressors via rapid compensation of pressure imbalance between both compressors, and a control method using the same.
2. Description of the Related Art
Generally, an air conditioner comprises a compressor, a condenser, a capillary tube, and an evaporator. In operation, if the compressor compresses a gaseous refrigerant, the compressed high-pressure gaseous refrigerant is fed into the condenser to be liquefied therein. Subsequently, the condensed high-pressure liquid refrigerant is instantaneously vaporized in the evaporator (hereinafter, referred to as an indoor heat exchanger, more briefly, indoor unit) while passing through the capillary tube having a small diameter. While absorbing heat during vaporization, the refrigerant causes a drop in temperature to produce cool air, so that the cool air is discharged to a room for air conditioning.
The vaporized refrigerant, after that, passes through the compressor and the condenser, so that it discharges heat while being liquefied. In this way, the refrigerant is continuously used to carry out the above-described air conditioning operation.
FIG. 1 is a configuration diagram illustrating a conventional air conditioner having first and second compressors. FIG. 2 is a configuration diagram illustrating another conventional air conditioner having first and second compressors and first and second indoor units. FIG. 3 is a diagram illustrating a conventional capacity control method using first and second compressors.
Referring to FIG. 1, the conventional air conditioner comprises: first and second compressors 12 and 14 for compressing a gaseous refrigerant to obtain a high-temperature and high-pressure refrigerant; first and second discharge pipes 16 and 18 used to deliver the refrigerant compressed in the first and second compressors 12 and 14; first and second check valves 20 and 22 provided downstream of the first and second discharge pipes 16 and 18, respectively, to prevent backflow of the refrigerant; a condenser 24 for condensing the refrigerant, having passed through the first and second check valves 20 and 22, to obtain a low-temperature and high-pressure liquid refrigerant; an expansion valve 26 used to rapidly expand the refrigerant delivered from the condenser 24 to obtain a low-temperature and low-pressure refrigerant; an indoor unit 2 for transferring heat of indoor air to the low-temperature refrigerant having passed through the expansion valve 26 to produce cool air to be discharged into a room; and first and second suction pipes 4 and 6 used to diverge the refrigerant vaporized in the indoor unit 2. The first and second compressors 12 and 14 and the condenser 24 are mounted in an outdoor shell B, while the expansion valve 26 and the indoor unit 2 are mounted in an indoor shell A.
Assuming that the total compression capacity of the refrigerant is 100%, the first compressor 12 has a partial refrigerant compression capacity of 40%, and the second compressor 14 has a partial refrigerant compression capacity of 60%.
Now, the operation of the above-described conventional air conditioner having the single indoor unit will be explained. First, when a user initiates full-capacity operation of the air conditioner, both the first and second compressors 12 and 14 are simultaneously operated, so that the refrigerant, suctioned through both the first and second suction pipes 4 and 6, is compressed and discharged into the first and second discharge pipes 16 and 18.
Subsequently, the compressed refrigerant is delivered into the condenser 24 by way of the first and second check valves 20 and 22 provided at the first and second discharge pipes 16 and 18 without a backflow risk. Thereby, the refrigerant is liquefied in the condenser 24, and then, passes through the expansion valve 26 and the indoor unit 2, to supply cool air into a room.
On the other hand, when the air conditioner is operated in a power-saving mode, only one of the first and second compressors 12 and 14 is selectively operated, thereby supplying weakly cool air into a room.
Referring to FIG. 3 illustrating an automatic operation mode of the air conditioner, the above-described selective operation of both the compressors is controlled based on a temperature difference (DT) between room temperature and the set temperature of the air conditioner.
Since the temperature difference (DT) corresponds to a load capacity of the indoor unit 2, the following definition is obtained.Load capacity of Indoor unit (DT)=Room Temperature (RT)−Set Temperature of Air Conditioner (ST)
If the temperature difference (DT) between the room temperature and the set temperature increases beyond a first preset value (DTS1), the full-load compression capacity of 100% is determined, causing both the first and second compressors 12 and 14 to be simultaneously operated. If the temperature difference (DT) falls between the first preset value (DTS1) and a second preset value (DTS2), a partial-load compression capacity of 60% is determined, causing only the second compressor 14 to be operated. Also, if the temperature difference (DT) falls between the second preset value (DTS2) and a third preset value (DTS3), a partial-load compression capacity of 40% is determined, causing only the first compressor 12 to be operated.
The control procedure as stated above minimizes the temperature difference (DT) between the room temperature and the set temperature of the air conditioner, keeping the room temperature (RT) at a desired appropriate value.
Referring to FIG. 2 illustrating another conventional air conditioner having first and second compressors and first and second indoor units, the configuration of FIG. 2 is substantially identical to that of FIG. 1 except for the use of first and second selector valves 8 and 10. The first selector valve 8 serves to selectively deliver the refrigerant condensed in the condenser 24 to the first indoor unit 2a, and the second selector valve 10 serves to selectively deliver the refrigerant condensed in the condenser 24 to the second indoor unit 2b. Here, the first indoor unit 2a is of a room air conditioner (RAC) type, and the second indoor unit 2b is of a package air conditioner (PAC) type.
For example, as the first indoor unit 2a, which is mounted in a bedroom, and the second indoor unit 2b, which is mounted in a living room, are selectively operated, the first and second compressors 12 and 14 are also able to be selectively operated in accordance with a given load capacity.
In this case, the expansion valve 26 of the RAC type first indoor unit 2a may be located upstream of the first selector valve 8.
When the air conditioner of FIG. 2 is operated in the automatic operation mode as shown in FIG. 3, the operation of the first and second compressors 12 and 14 is controlled in accordance with the load capacities of the first and second indoor units 2a and 2b. 
Here, the load capacity (DT) of the first indoor unit 2a is defined as the room temperature (RT) minus the set temperature of the air conditioner (ST), and similarly, the load capacity of the second indoor unit 2b is defined as the room temperature (RT) minus the set temperature of the air conditioner (ST). Thus, the operation of the first and second compressors 12 and 14 is controlled in accordance with the total load capacity (DT) of the first and second indoor units 2a and 2b. 
If the total load capacity (DT) increases beyond a first preset value (DTS1), the full-load compression capacity of 100% is determined, causing both the first and second compressors 12 and 14 to be simultaneously operated. If the total load capacity (DT) falls between the first preset value (DTS1) and a second preset value (DTS2), a partial-load compression capacity of 60% is determined, causing only the second compressor 14 to be operated. Also, if the total load capacity (DT) falls between the second preset value (DTS2), and a third preset value (DTS3), a partial-load compression capacity of 40% is determined causing only the first compressor 12 to be operated.
However, the multistage type air conditioners as stated above have a problem in that excess electricity is used to start operation of the first and second compressors 12 and 14. This results in significant degradation of energy consumption efficiency because of repetitive starting-operation/stop-operation of the compressors.
Also, the first and second discharge pipes 16 and 18 between the first and second compressors 12 and 14 and the first and second check valves 20 and 22 are affected by a high-pressure, whereas entrance ends of the first and second compressors 12 and 14 and the first and second suction pipes 2 and 6 are affected by a low-pressure. In such a pressure imbalance condition, it is very difficult to restart operation of the first and second compressors 12 and 14. Thus, there exists a need for a pressure equalizing time, and this prevents prompt operation of the compressors.
Furthermore, in the case of conventional air conditioners designed to achieve an increase in energy consumption efficiency via frequent repetitive starting-operation/stop-operation of the plurality of compressors, they inevitably confront a liquid backflow phenomenon due to a pressure difference caused during operation of the compressors. The liquid backflow phenomenon causes frequent starting-operation/stop-operation of the compressors, resulting in compressor wear and degrading of the reliability of the compressors.